1. Field of the Invention
This invention relates to an electrophotographic photosensitive medium having a photoconductive insulating binder layer, and more particularly it relates to an electrophotographic photosensitive medium having an electrically insulating layer on the surface of a photoconductive insulating binder layer.
2. Description of the Prior Art
An electrophotographic photosensitive medium having a photoconductive layer formed on a base and an electrically insulating layer (usually light-transmissible) on the photoconductive layer is useful and effective for electrophotographic process in which charging is applied to the surface of the electrically insulating layer to give electric charges thereto and an electrostatic latent image is formed in accordance with the electric charges. Particularly, the photosensitive medium is capable of exhibiting remarkable effect in the electrophotographic process as disclosed in U.S. Pat. Nos. 3,666,363 and 3,734,609.
However, in the above-mentioned electrophotographic process in which charging is applied to the surface of the electrically insulating layer to give electric charges thereto and an electrostatic latent image is formed in accordance with the charges, the electrostatic latent image-forming characteristic of the photosensitive medium depends greatly upon not only the property of the electrically insulating layer itself but also the interface state between the electrically insulating layer and the photoconductive layer. For example, in case that the photoconductive layer is of N-type semiconductor characteristic, when a positive charging is effected, positive (+) electric charges are given onto the electrically insulating layer while the electric charges of the opposite polarity, that is, negative (-) charges are simultaneously injected from the base side and trapped in the vicinity of the interface between the electrically insulating layer and the photoconductive layer. The negative charges thus trapped and the positive charges on the electrically insulating layer attract each other through the electrically insulating layer, which results in substantial charging of the positive charges on the electrically insulating layer.
At that time, if the photoconductive layer is uneven in its surface or of porous state, the charges are ununiformly trapped, and therefore the corresponding charges on the electrically insulating layer also become ununiform in their distribution. Thus, an electrostatic latent image to be formed in accordance with such charges is not said to be excellent, and in an extreme case it is not practical for use, which is an inconvenience of the conventional photosensitive media.
Further, if the photoconductive layer is uneven in its surface or of porous state as mentioned above, it becomes impossible to form an electrically insulating layer of a uniform thickness and smooth surface on the photoconductive layer. As the result, the same inconvenience as in the foregoing is caused. The cause of the problem as above rises remarkably in forming the photoconductive layer by means of a dispersion of the photoconductive material in the binder.
That is, when the photoconductive layer, in other words, the photoconductive insulating binder layer is formed by means of a dispersion prepared by dispersing the photoconductive material in the binder, the weight ratio of the binder to the photoconductive material is usually 50% by weight or below, and due to this, it is impossible that the whole air gap between or among the photoconductive material particles in the photoconductive insulating binder layer is perfectly filled up with the binder. As the result, a great number of fine interstices are formed in the photoconductive insulating binder layer, and therefore, such photoconductive insulating binder layer becomes porous and uneven surface. It would be considered to be well that a larger amount of the binder is used in order to form the photoconductive insulating binder layer which does not contain fine interstices and does not have uneven surface. However, if the weight ratio of the binder to the photoconductive material exceeds 50% by weight, the photoconductive characteristic of the formed photoconductive insulating binder layer is decreased and thus the characteristic of the electrophotographic photosensitive medium is deteriorated.
In consideration these points, when the electrically insulating layer is provided on the surface of the photoconductive insulating binder layer which has been formed by using 50% by weight or below of the binder to avoid the deterioration of the electrophotographic photosensitive medium characteristics, the electrically insulating layer--forming insulating material permeates into the interstices contained in the photoconductive insulating binder layer and the interstices are filled up with the insulating material, which causes remarkable decrease in the photoconductive characteristic of the photoconductive insulating binder layer. In addition to this, also the solvent for the insulating material of the electrically insulating layer permeates into the above-mentioned interstices to thereby cause the reduction in the binding ability of the binder in the photoconductive insulating binder layer. As the result of these inconveniences, the characteristic of the electrophotographic photosensitive medium is extremely deteriorated. Further, air bubbles are liberated from the interstices contained in the photoconductive insulating binder layer at the time of forming the electrically insulating layer and forced to get into the insulating layer so that dielectric strength in the portion where the air bubbles are present is lowered and pin holes are liable to be formed. Due to the phenomenon, the foregoing electrification state is remarkably deteriorated.
On the other hand, when the surface of the photoconductive insulating binder layer is in a concave and convex form, it cannot be denied that the thickness of the electrically insulating layer to be formed becomes ununiform and the surface smoothness thereof also becomes poor.
In order to remove the above-mentioned problems, it is required to provide a thin protecting layer (hereinafter called "clearcoling layer") on the photoconductive insulating binder layer by means of a coating liquid which is prepared by dispersing a material which does not affect adversely the binder of the photoconductive insulating binder layer and also is not affected adversely by the insulating material of the electrically insulating layer or the solvent therefor in a solvent which does not affect adversely the binder of the photoconductive insulating binder layer, in other words, does not change chemically and physically the photoconductive insulating binder layer.
An electrophotographic photosensitive medium having such a clearcoling layer as mentioned above is disclosed in United States Ser. No. 391,761 filed Aug. 27, 1973 and German Patent Publication (DAS) No. 2,344,777.
The photosensitive medium disclosed in the United States patent application is of such a structure that a clearcoling layer composed of an organic solvent-resistant, water-soluble and cohesive material is provided on the surface of a photoconductive layer formed of a dispersion of inorganic photoconductive particles in a water-insoluble binder and an electrically insulating layer is further provided on the clearcoling layer. In the invention of the above-mentioned United States Application, it cannot be denied that the clearcoling layer-forming material is restricted to some extent, that is, it has to be selected from the materials capable of satisfying the restricted requirement that they should be organic solvent-resistant, water-soluble and cohesive.
Therefore, an improvement in the electrophotographic photosensitive media having the above-mentioned clearcoling layer is intended in this invention.